My Reformation Study Bible, ed. by R.C. Sproul, Sr., (Nashville, TN: Thomas Nelson, 1995) articulates the basics of the Christian doctrines of “Justification and Merit” (p. 1852) (which also pertains to “Salvation.”) Therein it notes:
The doctrine of justification, the stormcenter of the Reformation, was for Paul the heart of the gospel (Rom. 1:17; 3:21-5:21; Gat. 2:15-5:1), shaping his message (Acts 13:38-39) and his devotion (2 Cor. 5:13-21; Phil. 3:14-14). Though other New Testament writers affirm the same doctrine in substance, the terms in which Protestants have affirmed and defended it for almost five centuries are drawn primarily from Paul.
Justification is God’s act of pardoning sinners and accepting them as righteous for Christ’s sake. In it, God puts permanently right their previously estranged relationship with Himself. This justifying sentence is God’s bestowal of a status of acceptance for Jesus’ sake (2 Cor. 5:21).
God’s justifying judgment seems strange, for pronouncing sinners righteous may appear to be precisely the king of unjust action by a judge that God’s own law forbids (Deut. 25:1; Prov. 17:15). Yet it is a just judgment, for its basis is the righteousness of Jesus Christ. As “the last Adam” (1 Cor. 15:45), our representative head acting on our behalf, Christ obeyed the law that bound us and endured the punishment for lawlessness we deserved, and so “merited” our justification. Our justification is on a just basis (Rom. 3:25-26; 1 John 1:9), with Christ’s righteousness reckoned to our account (Rom 5:18-19).
God’s justifying decision is in effect the judgment of the Last Day regarding where we will spend eternity, brought forward into the present and pronounced here and now. It is a judgment on our eternal destiny; God will never go back on it, however much Satan may appeal against the verdict (Zech. 3:1; Rom. 8:33-24; Rev. 12:10). To be justified is to be eternally secure (Rom. 5:1-5; 8:30).
The necessary means of justification is a personal faith in Jesus Christ as crucified Savior and risen Lord (Rom. 5:23-25; 10:8-13). Faith is necessary because the meritous ground of our justification is entirely in Christ. as we give ourselves to faith to Jesus, Jesus gives us His gift of righteousness, so that in the very act of “closing with Christ,” as older Reformed teachers put it, we receive the divine pardon and acceptance we can find nowhere else (Gal. 2:15-16; 3:24).